Magnets having one easy direction of magnetization



Nov. 22, 1960 J. s. KOUVEL EIAL MAGNETS HAVING ONE EASY DIRECTION OF MAGNETIZATION Filed Dec. 1, 1958 Fig.

MAX

Fig. 3.

by The/r Attorney.

MAGNETS HAVHNG UNE EASY DIRECTIQN OF MAGNETIZATION James S. Kouvel, Albany, and Charles D. Graham, .lr.,

Schenectady, N.Z., assignors to General Electric Company, a corporation of New York Filed Dec. 1, 1958, Ser. No. 777,324

'3 Claims. (Cl. 143-103) This invention relates to permanent magnets and more particularly to permanent magnets having one easy direction of magnetization and to a method for producing such magnets.

Previously known magnetic materials have been most readily magnetized along one particular axis and have been equally readily magnetizable in either direction along that axis. The magnetic properties of these materials have been measured and evaluated by several methods, perhaps the best known being the graphical representation of the hysteresis loop obtained when a magnetic field is applied to the magnetic material in such a manner as to cyclically reverse polarity. Since magnetic properties have been symmetrically reversible, both quantitatively and qualitatively with respect to the axis of magnetization, they also have at least two stable positions in a strong magnetic field.

While existing magnetic materials have generally proven acceptable for use in most situations, the increasing use of electronic and electrical apparatus under unusual environmental conditions has resulted in a need for components having particular properties when subjected to such unusual conditions. For example, various parts of missile guidance or propulsion systems must be able to perform reliably at extreme temperatures. Development of these and other technological areas has resulted in recognition that a magnetic material having one easy direction of magnetization would be of value, particularly where this property could be utilized at low temperatures.

A principal object of this invention is to provide magnets having one easy direction of magnetization.

Another object of this invention is to provide aprocess for producing magnets having one easy direction of magnetization.

Other objects and advantages will be in part obvious and in part explained by reference to the accompanying specification and drawings.

Fig. 1 shows a hysteresis loop illustrating the magnetic characteristic of the usual type of permanent magnet;

Fig. 2 shows a shifted hysteresis loop illustrating, the magnetic properties of a magnet made according to the present invention.

Fig. 3 shows a hysteresis loop illustrating the magnetic properties of a magnet made according to the present invention but without a magnetic field being applied.

Generally, this invention concerns permanent magnets and processes for producing permanent magnets having one easy direction of magnetization, which magnets com: prise bodies made of (a) alloys consisting of from'about 20 to 45 weight percent manganese combined with one or some combination of the metals, iron, nickel and cobalt, other than the iron-manganese binary alloys, and (b) alloys consisting of about 75 to 85 weight percent iron, balance substantially all aluminum.

The process generally comprises preparing a suitable ice alloy body and cooling the quenched alloy in a magnetic field to a temperature sufiiciently low to create the one easy direction of magnetization within the alloy body.

It has been mentioned previously that existing alloy magnets have at least two easy directions of magnetization and also have two equivalent stable positions in an applied magnetic field.

A hysteresis loop 10 characteristic of these magnets is shown in Fig. l, in which the magnetizing field H, in oersteds, is the axis of the abscissa and the magnetization or M, in gauss, is the axis of the ordinate. As the field is increased from 0 to higher values of H in what for convenience may be-referred to as the positive direction, the magnetization (M) of the material reaches a maximum value +M for a given field. If the field +H is removed, the value of |M decreases along the demagnetization curve 11 to +M If then a field H of reverse polarity is applied, the magnetization of the ma-- terial continues to decrease along curve 11 and crosses the H axis at the value H,,, the magnetization finally reach-' ing H as the field is further increased in the negative direction. If the negative field is removed, the magnetization of the material correspondingly drop to -M which is numerically equal to -lM Application of the positive field, then causes the magnetization to move along the magnetization curve 12 and cross the H axis at the value +H which is numerically equal to H.,. As the positive field is increased, the magnetization of the material rises to the value +M The magnetization values M which were used in the present case in place of the flux density, B, to plot the hysteresis loops, are equal to the flux density (B) minus the magnetizing field (H), divided by 4 pi, as indicated by the formula M==(BH)/41r.

Alloys which can be used, according to the present invention, to produce magnets having one easy direction of magnetization fall generally into two categories. The first category includes alloys containing between 20 and 45 percent manganese and the balance a metal selected from the group consisting of iron, nickel, cobalt and combinations thereof, other than iron-manganese binary al loys. The iron-manganese binary alloys cannot be processed to produce satisfactory results. The second gen eral category of alloys are iron base alloys containing 75 to weight percent of iron and the balance substantially all aluminum.

Fig. 2 shows a hysteresis loop 15 which is characteristic of the magnets of the present invention. Here the loop is shifted toward the negative direction of field, the material thus being more easilv magnetized in the posi-v tive direction. The maximum fields -H and +H are quantitatively the same, 8000 oersteds, but the ma netization values M are both positive and the field strengths H are both negat ve. For purposes of discussion, the magnetization and field values found on the de-' magnetization curve 16 of the loop are designated M and H respectively, and the corresponding values on the magnetization curve 17 are designated M and H respectively. 1

To illustrate more ful y the type of alloys which can be processed to de elop the desired properties, several examples follow. First, alloys containing from about 20 to about 35 percent manganese, balance substantially all nickel can be processed to develop the pro erties resulting in a hysteresis loop. Table 1 sets forth specific alloy compositions within the previously stated ranges and shows the magnetization and field values obtained at 4.2 K. The t ble also sets forth the approximate temperature at which the shifted loop effect appears when, the various alloys are cooled.

Table 1 shifted hysteresis loops. In the case of the manganese containing alloys, that is those alloys made up of man- Comrosi. N ganese and one or more oil the fmember; 1iror;i Mikel; 1011, Weight lTaX- cobalt, and combinations t ereo it is et t at or Percent MMEMU) MMEMU) HcwE) HCAOE) 135} 5 atomic and magnetic heterogeneity must exist to develop Ni Mn the one easy direction of magnetization. That is, some degree of disorder should exist to bring about the shlfted 80 +260 +240 20 hysteresis loop characteristic, although complete disorder 75 +95 +95 -500 -4co 50 is probably not essentlal as some ordered regions may 52 22 3"; 31 1583 3:338 38 10 exist in almost all systems if only on a statistical basis. As generally understood, a disordered solid solution Additional alloys are those a up of 25 percent mam alloy is one in which the atoms of the various constituent ganese, 19 to 371/2 percent iron and a balancg Substam metals of an alloy are randomly distributed through the tially all nickel. Some specific compositions within these alloy lattlce- Conversely, an Ordered Solid SDIUIIOII compositional ranges d having d i d magnetic propa is one in which the atoms of the different metals repeat erties, are found in the following Table II. in lattice position according to a definite pattern.

Table II Composition, Weight Max.

Percent Temp.

Mait M Mn2( c1( c2( v g g Ni Fe Mn I 56 19 25 +235 +235 700 -6c0 so 50 25 25 +145 +145 -1, 300 1, 150 60 37.5 37.5 25 +53 +51 -4,500 -3. 100 45 Cobalt may be totally or partially substituted for nickel The compositions in which ordering must be considered and combined with manganese to produce alloys which, are principally those in which the metals are combined when properly processed, have the desired one easy direcin stoichiometric proportions. For example, ordering tion of magnetization. The cobalt is added in amounts would ordinarily occur in the compositions Ni Mn or ranging from 55 to 75 percent in the binary alloys and NiMn. Of course, it is to be understood that the discusin amounts up to about 75 percent in the nickel-cobaltsion concerning the order or disorder of the various alloys manganese ternary alloys having 25 percent manganese. is advanced as a possible explanation of why the effect is Table III lists the magnetic characteristics of some of seen in these alloys and that other factors may be equalthese alloy compositions. It will be noted, that in the ly important. alloys where cobalt has been substituted for nickel, the Broadly, the process involves preparing a magnet body field strength (H and H are smaller and also that of a suitable alloy such as one of those previously disthe temperatures at which the shifted loop characteristic closed, quenching the body from an elevated temperadisappears are generally somewhat lower. ture to retain some degree of atomic disorder and cooling Table III the body to below about 80 K. in a magnetic field. The temperature to which the body must be cooled in a magnetic field must be equal to or lower than the temg Sgfi NHL perature at which the shifted hysteresis loop effect ap- Percent MmOE U; MME J c1( cz(OE) Temp. pears to obtain the one easy direction of magnetization. In order to obtain optimum properties, the field should 00 M11 he maintained down to the lowest temperature to which the body is cooled. Examples of such temperatures are 2% dig g 28 2 set forth in Tables I through IV. +2 0 I 3 0 3 In those alloys where orderingis anticipated, the mag- 55 10 net body should be heated to a temperature above 700 C., preferably about 900 C., and held for a sufiicient As a final compositional illustration, iron and aluminum amount of time to insure that disorder is achieved, for can be combined in amounts from 75 to 85 weight percent 55 example about 12 hours. Once disordered, the metal, is iron and the balance aluminum to obtain an alloy having quenched rapidly to room temperature to retain disone easy direction of magnetization. Here, as shown in ordering as cooling progresses through the lower tempera- Table IV, the shift is quite large for some alloy compositure ranges. The disordered material is then cooled tions although the temperature at which the effect during further to some temperature below about 80 K., in a cooling appears is generally lower than that for the alloys 6 magnetic field, the exact temperature depending upon the containing manganese. particular alloy, as already noted.

Table IV Where ordering is not a material consideration, heating of the body to an elevated temperature and subsequent Commsb quenching to room temperature need not be effected, alon, Atomic though optimum disorder is assured if these steps are Percent MIMEMU) mtE U) c1( c2(OE) Temp. included.

1 ,23 It is felt that the present magnets, as processed, acquire Fe A1 their unique behavior by virtue of ferromagnetic and antiferromagnetic regions present within the body. The cougg f; 4:583 fi88 18 pling effect between the ferromagnetic and anti-ferromageo 40 +2 +2 44,700 4,400 1 netic regions causes the one easy direction of magnetization. Therefore, during cooling of the body from room As has already been stated, the treatment to which the temperat it is essential that it be Subjected to a alloys must be subjected is important in achieving magnetic field, one on the order of 5000 oersteds normally nets having the magnetic properties evidenced by the being sufficient to properly orient the ferromagnetic moments in any of the alloys. Of course, lesser fields can be used on some of the alloys and still obtain maximum alignment of the magnetic moments within the magnet. Although the magnetic field need only be applied from just above that temperature at which the shifted loop effect begins, as a matter of expedience it is normally easier to apply the field continuously during the entire cooling treatment.

The elfect of cooling the body from room temperature to below the critical temperature without using a mag netic field can be shown by referring to loop 20 of Fig. 3 of the drawings. This loop Was obtained from the same material used to obtain curve 15, the difierence resulting from the absence of an applied magnetic field during cooling.

The processing of the iron-aluminum alloy is generally the same as that of the previously discussed alloys, although the presence of disorder is not felt to be a consideration as the alloys are normally ordered. As an example, an iron-aluminum alloy containing approximately 17 percent aluminum was annealed for 20 hours at 900 C., quenched from 700 C. in Water, and then cooled from room temperature to 1.8 K. in a magnetic field of 5000 oersteds. The measurements were made at the point 1.8 K.; the magnetization M was measured in the direction of the field applied during cooling. The hysteresis loop crossed the H axis at plus 200 and minus 700 oersteds, thereby indicating that asymmetry of the hysteresis loop had taken place.

Thus, the present invention has provided a magnet which is totally metallic containing ferromagnetic and anti-ferromagnetic regions which bring about the one easy direction of magnetization.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In the process of producing a permanent magnet having one easy direction of magnetization at temperatures below about 80 K., the steps comprising preparing a body composed of an alloy selected from the group consisting of (a) alloys containing from about 20 to 45 weight percent manganese, balance a metal from the group consisting of iron, nickel, cobalt, and combinations thereof, other than iron-manganese binary alloys and (b) alloys containing from about to 85 weight percent velop the one easy direction of magnetization within the g body.

2. In the process of producing a permanent magnet having one easy direction of magnetization at temperatures below about K. with a body composed of an alloy consisting essentially of from about 20 to 45 weight percent manganese, and the balance a metal selected from the group consisting of iron, nickel, cobalt, and combinations thereof, other than iron-manganese binary alloys, the steps comprising cooling the alloy body to a temperature below about 80 K., and subjecting the body to a magnetic field during cooling to align the magnetic moments of the ferromagnetic regions and develop the one easy direction of magnetization within the body.

3. A process for producing a permanent magnet having one easy direction of magnetization at temperatures below about 80 K., comprising preparing an alloy ingot consisting essentially of from about 20 to 45 weight percent manganese, and the balance a metal selected from the group consisting of iron, nickel, cobalt, and combinations thereof, other than iron-manganese binary alloys, heating the body to above about 700 C., quenching the alloy body, cooling the alloy body to a temperature below about 80 K., and subjecting the body to a magnetic field during cooling to align the magnetic moments of the ferromagnetic regions and develop the one easy direction of magnetization within the body.

References Cited in the file of this patent UNITED STATES PATENTS 1,668,115 Gumlich May 1, 1928 2,133,291 Gordon Oct. 18, 1938 2,295,082 Jonas Sept. 8, 1942 2,382,650 Nesbitt Aug. 14, 1945 2,859,143 Nachman et a1. Nov. 4, 1958 FOREIGN PATENTS 280,893 Switzerland May 16, 1952 

1. IN THE PROCESS OF PRODUCING A PERMANENT MAGNET HAVING ONE EASY DIRECTION OF MAHNETIZATION AT TEMPERATURES BELOW ABOUT 80*K., THE STEPS COMPRISING PREPARING A BODY COMPOSED OF AN ALLOY SELECTED FROM THE GROUP CONSISTING OF (A) ALLOYS CONTAINING FROM ABOUT 20 TO 45 WEIGHT PERCENT MANGANESE, BALANCE A METAL FROM THE GROUP CONSISTING OF IRON, NICKEL, COBALT, AND COMBINATIONS THEREOF, OTHER THAN IRON-MANGANESE BINARY ALLOYS AND (B) ALLOYS CONTAINING FROM ABOUT 75 TO 85 WEIGHT PERCENT IRON BALANCE SUBSTANTIALLY ALL ALUMINUM, COOLING THE ALLOY BODY TO A TEMPERATURE BELOW ABOUT 70*K.. AND SUBJECTING THE BODY TO A MAGNETIC FIELD DURING COOLING TO ALIGN THE MAGNETIC MOMENTS OF THE FERROMAGNETIC REGIONS AND DEVELOP THE ONE EASY DIRECTION OF MAGNETIZATION WITHIN THE BODY. 